Biological defense strategies have evolved to protect organisms from invasion by other species. Microbial infection response systems include oxidative and non-oxidative mechanisms, utilizing compounds that are enzymatically synthesized in cells and peptides that are single gene products.
Anti-microbial peptides constitute an oxygen-independent host defense system found in organisms encompassing many taxonomic families. One major class of anti-microbial peptides can be sequence-defined by conserved cysteine residue patterns and are termed defensins. Mammalian defensins, derived from skin, lung and intestine, exhibit antibiotic activity against a wide variety of pathogens, including gram-positive and gram-negative bacteria, fungi (e.g. Candida species) and viruses. See, for example, Porter et al., Infect. Immun. 65(6): 2396–401, 1997.
The amphipathic character of the defensin peptides appears to be the key to the general mechanism of microbial attack, i.e., by creating pores, or “boring” through the cell wall. In addition, Daher et al., J. Virol. 60(3): 1068–74, 1986, reported that enveloped viruses, including herpes simplex types 1 and 2, cytomegalovirus and influenza virus (A/WSN), among others, were inactivated by incubation with human neutrophil peptide (HNP-1) and speculated that the binding of defensin molecules to viruses impairs the virus' ability to infect cells.
The defensin family of anti-microbial peptides can be divided into two major subclasses based on two distinct consensus sequences. See, for example, Martin et al., Journal of Leukocyte Biology 58: 128–36, 1995. The first defensin subclass, classic defensins, represented by HNPs are stored in the so-called large azurophil granules of neutrophils and macrophages and attack microorganisms that have been phagocytosed by these cells. The amino acid sequence of HNPs is consistent with a predicted disulfide bridging that is distinct from that of the β-defensin subclass. Epithelial cells can also be a source of defensins, and these cells appear to secrete these peptides into the external, extra-cellular environment. In the mouse, for example, Paneth cells of the small intestine and proximal colon, secrete defensin-like peptides, called cryptidins, into the lumen. See, for example, Ouellette and Selsted, The FASEB Journal 10: 1280–9, 1996.
β-defensins, the second major defensin subclass, include peptides found in bovine lung (e.g., BNBD-bovine neutrophil β-defensins) as well as a secreted form (TAP—tracheal anti-microbial peptide). See, for example, Selsted et al., J. Biol. Chem. 268(9): 6641–8, 1993. Two human β-defensins have been reported. SAP-1 was isolated from human psoriatic skin, and hBD-1 was found in low concentrations in human blood filtrate. See, for example, Bensch et al., FEBS Lett. 368(2): 331–5, 1995). The amino acid sequence of these human β-defensins is most similar to the bovine BNDPs and TAP. See, for example, Harder et al., Nature 387: 861, 1997, wherein SAP-1 is designated hBD-2.
Other than the conserved cysteine residues the defensin family is quite sequence divergent. It is possible that the variant amino acid positions may be related to the site or conditions of activity or to the spectrum of pathogens attacked by a particular defensin.
In addition to anti-microbial activities, particular defensins exhibit metabolically sensitive cytotoxic activity (Lichtenstein et al., Blood 68: 1407–10, 1986 and Sheu et al., Antimicrob. Agents Chemother. 28: 626–9, 1993), alter the response of adrenal cortical cells to ACTH (Zhu et al., Proc. Natl. Acad. Sci. (USA) 85(2): 592–6, 1988) and have specific chemotactic activity for human monocytes (Territo et al., J. Clin. Invest. 84(6): 2017–20, 1989). Recruitment of monocytes by neutrophils may, in part, be mediated by neutrophilic defensins and suggests a pro-inflammatory activity for these peptides in addition to their anti-microbial effects. Also, a decrease in defensin mRNA level has been demonstrated in SPG (specific granule disease). See, for example, Tamura et al., Japan. Int. J. Hematol. 59(2): 137–42, 1994. Higazi et al., J. Biol. Chem. 271(3): 17650–5, 1996, suggested that plasminogen bound to fibrin in the presence of defensin may be less susceptible to activation by tPA.
Moieties having anti-microbial, immunostimulatory, pro-inflammatory and other properties of defensins are sought. The present invention provides such polypeptides for these and other uses that should be apparent to those skilled in the art from the teachings herein.